Large brains are humans' most distinctive anatomical feature. Our brains are about four times bigger than chimpanzees' and gorillas' brains.

Brains use twenty times the calories of muscles at rest. Brains require maintaining a constant temperature. Large brains are easily injured, and make childbirth difficult. Intelligence has many costs, yet doesn't directly help an animal survive (e.g., a big brain doesn't make you run faster or survive colder weather).

Our ancestors' brains began to enlarge about two million years ago. In evolutionary time, two million years is short. Why our ancestors rapidly evolved large brains—specifically, a large, uniquely human cerebral cortex—is the central question of human evolution.

The Triune Brain

The oldest, deepest, and smallest area is the reptilian brain.[2] The reptilian brain controls the heart, lungs, and other vital organs. It enables aggression, mating, and reaction to immediate danger.

Mammals evolved the limbic system. This is the middle layer of our brains, surrounding the reptilian brain. The physiological features unique to mammals are in the limbic brain, e.g., the hypothalamus system for keeping us warm.

The most common reaction a reptile has to its young is indifference; it lays its eggs and walks (or slithers) away. Mammals form close-knit, mutually nurturant social groups-families-in which members spend time touching and caring for one another. Parents nourish and safeguard their young, and each other, from the hostile world outside their group. A mammal will risk and sometimes lose its life to protect a child or mate from attack. A garter snake or salamander watches the death of its kin with an unblinking eye.[3]

The cerebral cortex (or neocortex) is the newest, outermost area of our brains. The oldest mammals, e.g., opossums, have only a thin layer of cerebral cortex. Rabbits have a little more, cats a bit more. Monkeys have a substantial cerebral cortex. Humans—and only humans—have an enormous cerebral cortex.[4]

The human reptilian brain and limbic system is similar is size and structure to other animals. I.e., our ancestors evolved a huge cerebral cortex, while the older brain areas didn't change.

The cerebral cortex learns new things. Animals with little or no cerebral cortex act only as their genes program them to act. Animals with a cerebral cortex can find new foods, survive in new environments, or change their mating tactics to improve reproductive success.

The human cerebral cortex goes beyond learning new foods and survival skills. Our brains can think in abstractions. We communicate via symbols (e.g., language), consider the past and future, and sacrifice our personal interests not only for our families (as other mammals do) but also for ideas (e.g., honor and country).

Conflicts between brain areas lead to relationship difficulties. In a conflicted brain, the older area wins. In contrast, an individual with an integrated brain—i.e., who uses his or her whole brain—solves relationship problems.

Ontogeny Recapitulates Phylogeny

A child's development mimics its species' evolution.

Infants live in their reptilian brains. They eat, breathe, crawl, sleep, etc.

Children live in their limbic brains. They feel emotions strongly. They use emotions to form relationships.

Adolescents live in their cerebral cortexes. They strive to become unique individuals. They quest to find abstract principles to live by.

Adult relationships invert childhood development. Men and women use cerebral cortex abstractions (e.g., gender roles) to attract opposite sex partners. If a couple then feels limbic brain emotionally connected "chemistry," they form a relationship. If the relationship goes well, sooner or later they're in bed, using their reptilian brains.

Love develops a child's limbic brain.[5] Unloved children fail to develop limbic brains capable of emotional intimacy. Such an individual can relate on a reptilian level—e.g., food, warmth, sex—or on a cerebral cortex level—e.g., excelling at accounting or the law—but have difficulty with intimacy.

Natural vs. Sexual Selection

In The Origin of Species (1859), Charles Darwin wrote that species evolve via random mutations. Environmental changes—e.g., changing food sources, predation, climate—favor one mutation over another. He called this process natural selection.

The conventional view is that our smarter, larger-brained ancestors invented tools, and then dominated their smaller-brained relations. The archaeological facts don't support this "man the toolmaker" hypothesis.

Our ancestors first used stone tools 2.5 million years, or 100,000 generations, ago.[6] This book has about 50,000 words. To refer to the first human as your "great-great-great…grandparent," you'd have to replace every word in this book with "great," and you'd need two books.

After one million years, or near the end of the first book, our ancestors' brains were more than double in size. Archaeologists can see slight improvements in their stone tools.[7]

500,000 years ago—halfway through the second book—our ancestors' brains were nearly as big as our brains. Our ancestors started using fire.[8] Fire enabled them to move from Africa to colder Europe and Asia.

50,000 years ago—eight pages from the end of the second book—our ancestors' brains reached modern size. Their stone tools became thinner and sharper. They carved small ornamental figurines from ivory, shell, and stone. They created beautiful cave paintings. They built the first ocean-going boats.[9]

5,000-10,000 years ago—the last page of the second book—our ancestors developed agriculture. Poor nutrition made farmers' bodies and brains smaller. They invented writing and metal tools. They invented the bow and arrow-a weapon that seems primitive to us.[10]

Our ancestors' brains enlarged before technological advances. Our ancestors' brains were ready for modern technology long before they invented it. Tool use was a spandrel or side effect of large brains. Something else drove human brain evolution.

Sexual Selection

In The Descent of Man (1871), Darwin wrote that natural selection failed to explain human evolution. Instead, he proposed an alternative theory. Species evolve when males and females select each other for certain qualities. He called this sexual selection. Biologists ignored this idea for over a century.[11]

Female mammals, in general, are more selective than males. Females in most mammal species do most of the work of producing and raising children. In contrast, fathering offspring is less work, so males aren't so choosy.

The exertion of some choice on the part of the female seems almost as general a law as the eagerness of the male.[12]

— Charles Darwin, The Descent of Man (1871)

Females choose males with features that make the males less able to survive.[13] E.g., a peacock's bright colors make him visible to predators, and his huge tail slows his escapes. His beautiful tail communicates to peahens that he's an especially fit individual, i.e., he's so fast that he can escape predators despite his heavy tail. Sexual selection is, in general, the opposite of natural selection.

What's Sexy About a Cerebral Cortex?

Humans' oversized brains could have evolved due to sexual selection. But what's sexy about an enlarged cerebral cortex? Women don't say, "Look at the cerebral cortex on that dude! I want to have his children!" Our ancestors must have instead been attracted to cerebral cortex behaviors.

Our cerebral cortexes enable many behaviors, e.g., speech and language. But what's striking about the cerebral cortex is how much of it is not dedicated to specific behaviors. The human cerebral cortex has billions of general-purpose neurons, capable of learning any new idea. Why were our ancestral mothers and fathers—unlike any other animals—sexually attracted to partners who could learn new ideas?

Monogamy and Lying

Most nonhuman mammal fathers have little or no involvement with their offspring.[14] Male gorillas kill infants fathered by other males. Male chimpanzees help all the youngsters in their group, but they don't know who fathered each child.

Human evolution may have begun when fathers helped raise their children, giving the children a survival advantage. Among hunter-gatherers today, children without fathers are more than twice as likely to die during childhood.[15]

Monogamy could cause a conflict between two reproductive strategies. A man could try to have sex with many women, risking rejection from women, violence from other men, or his fatherless children not surviving. Although initially more offspring might be conceived this way, such a man might father no surviving children.

Or a man could choose to be in a monogamous relationship, and actively raise his children. Such a man would father only a few children, but his children would likely survive and prosper.

A woman could have sex with a desirable (e.g., high-status, tall, strong, handsome) man, and risk competing women taking him from her. Or she could choose a stable, monogamous relationship with a less-desirable man whom no one other woman wanted.

Both men and women could have increased reproductive success by lying. A man's lie could be to promise or give the impression of commitment to woman, impregnate her, and then leave. Or a man "committed" to one woman could secretly be involved with a second woman, perhaps in a neighboring town or village.

A woman's lie might be to get pregnant by a physically desirable (but uncommitted) man, and then tell her committed (but less desirable) partner that the child is his.

Or a woman can lie to a less-desirable man that she'll marry him (i.e., keep him as an insurance policy), while dating more desirable men in hope that one will marry her.

Or a woman can lie to a desirable man, who's "committed" to another woman, that she only wants a short-term sexual relationship. She then leaves an earring in his bed for his wife to find. If she can break up his marriage, he might marry her.

Those are cerebral cortex activities. Effective liars also match their emotions to their lies. You catch lies when an individual's emotional state doesn't match his or her words. Effective lying requires integrating one's cerebral cortex with one's limbic brain.

A man or woman with a larger cerebral cortex, well-integrated with his or her limbic brain, is better able to sexually lie, and to catch sexual lies. Such men and women became our ancestors.